This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Chlamydia represents a group of gram negative intracellular pathogens that are known to cause various health problems in humans. It is generally accepted that Chlamydia-induced diseases are due to host inflammatory responses caused by chlamydial infection. Several chlamydial species carry a 7.5 Kb cryptic plasmid. The plasmid encodes eight putative open reading frames (pORF). pORF5 encodes Pgp3 which is a virulence factor that is mainly detected in the cytosol of Chlamydia infected cells 12 hours after infection. Pgp3 is secreted by all chlamydial species that have the cryptic plasmid. It interacts with the host cell signaling pathway including the activation of host inflammatory genes. The structure of Pgp3 can provide valuable information to help understand the activation of the inflammatory response and chlamydial pathogenesis.